Configuration of a ran based notification area for a user equipment in rrc inactive state

ABSTRACT

A communication system is disclosed in which a base station receives, from at least one further base station, information identifying at least one respective tracking area associated with at least one cell of each further base station. The base station also receives, from a core network, information identifying a registration area for a user equipment (UE). The base station defines, based on the received information identifying at least one respective tracking area and the received information identifying a registration area, a radio access network (RAN) based notification area for the UE.

CROSS REFERENCE

This application is a Continuation of PCT International Application No.PCT/JP2018/029637 filed on Aug. 7, 2018, which claims priority underU.S.C. § 119(a) to United Kingdom Patent Application No. 1712862.0 filedon Aug. 10, 2017, the disclosure of which is incorporated herein in itsentirety by reference.

TECHNICAL FIELD

The present invention relates to a communication system. The inventionhas particular but not exclusive relevance to wireless communicationsystems and devices thereof operating according to the 3rd GenerationPartnership Project (3GPP) standards or equivalents or derivativesthereof (including LTE-Advanced and Next Generation or 5G networks). Theinvention has particular although not exclusive relevance toimprovements related to the tracking of user equipment whilst in aninactive state.

BACKGROUND ART

The latest developments of the 3GPP standards are referred to as theLong Term Evolution (LTE) of Evolved Packet Core (EPC) network andEvolved UMTS Terrestrial Radio Access Network (E-UTRAN), also commonlyreferred as ‘4G’. In addition, the term ‘5G’ and ‘new radio’ (NR) referto an evolving communication technology that is expected to support avariety of applications and services. Various details of 5G networks aredescribed in, for example, the ‘NGMN 5G White Paper’ V1.0 by the NextGeneration Mobile Networks (NGMN) Alliance, which document is availablefrom https://www.ngmn.org/5g-white-paper.html. 3GPP intends to support5G by way of the so-called 3GPP Next Generation (NextGen) radio accessnetwork (RAN) and the 3GPP NextGen core network.

Under the 3GPP standards, an eNB in LTE (or gNB/NG-RAN node in 5G), isthe base station via which communication devices (user equipment or‘UE’) connect to a core network and communicate to other communicationdevices or remote servers. For simplicity, the present application willuse the term base station to refer to any such base stations and use theterm UE, user device, or UE to refer to any such communication device.The core network (i.e. the EPC in case of LTE) hosts functionality forsubscriber management, mobility management, charging, security, andcall/session management (amongst others), and provides connection forcommunication devices to external networks, such as the Internet.

Items of user equipment (UEs) might include, for example, mobilecommunication devices such as mobile telephones, smartphones, userequipment, personal digital assistants, laptop/tablet computers, webbrowsers, e-book readers and/or the like. Such mobile (or even generallystationary) devices are typically operated by a user. However, 3GPPstandards also make it possible to connect so-called ‘Internet ofThings’ (IoT) devices (e.g. Narrow-Band IoT (NB-IoT) devices) to thenetwork, which typically comprise automated equipment, such as variousmeasuring equipment, telemetry equipment, monitoring systems, trackingand tracing devices, in-vehicle safety systems, vehicle maintenancesystems, road sensors, digital billboards, point of sale (POS)terminals, remote control systems, and the like. Effectively, theInternet of Things is a network of devices (or “things”) equipped withappropriate electronics, software, sensors, network connectivity, and/orthe like, which enables these devices to collect and exchange data witheach other and with other communication devices. It will be appreciatedthat IoT devices are sometimes also referred to as Machine-TypeCommunication (MTC) communication devices or Machine-to-Machine (M2M)communication devices.

For simplicity, the present application refers generally to UEs in thedescription and it will be appreciated that the technology described canbe implemented on any communication devices (mobile and/or generallystationary) that can connect to a communications network forsending/receiving data, regardless of whether such communication devicesare controlled by human input or software instructions stored in memory.

Communication between UEs and base stations is controlled using a RadioResource Control (RRC) layer based on an RRC protocol as defined in thecurrent version of 3GPP TS 36.331. The RRC layer handles the controlplane signalling of Layer 3 (network layer) between UEs and the radioaccess network, and includes, amongst other things, functions forbroadcasting system information, paging, connection establishment andrelease, radio bearer establishment, reconfiguration and release,mobility procedures, and power control. In accordance with the currentversion of the RRC protocol, at any given time, a UE may operate eitherin an ‘RRC idle mode’ (in which no data communication takes place) or an‘RRC connected mode’ (in which data communication may take place betweenthe UE and its serving base station).

As UEs operating in the RRC connected mode move around in the areacovered by the communication system, they are handed over from one cell(i.e. operated by a base station) to another cell (operated by the sameor a different base station), depending on signal conditions and otherrequirements, such as requested quality of service, the type of serviceused, overall system load, and the like. Handover requires extensivesignalling between the UE and the base stations (old and new) and alsobetween the base stations and the core network as well.

On the other hand, whilst in the RRC idle mode, UEs are programmed toselect a ‘serving’ cell, having a good quality signal, to camp on sothat when new data is to be transmitted to/from these UEs, they canbenefit from favourable signal conditions. In the event that an idle UEdetects a new cell with better signal quality than the current servingcell, e.g. due to the UE changing its location, the UE can perform aso-called cell reselection procedure. However, an idle mode UE does notinform the network about the selected new cell as long as this cell iswithin the same ‘tracking area (TA)’ (i.e. a larger geographic areacomprising a pre-defined set of cells), because the radio networktransmits system information and UE specific paging messages within thewhole TA thus making it possible to contact and initiate communicationto/from the UE regardless of the current cell of the TA that the UEcamps on. A Tracking Area identity (TAI) is used to identify eachtracking area. The use of TAs has been extended by the so called“tracking area list concept” in which, when a UE registers with thenetwork, a core network node (e.g. a mobility management entity (MME)allocates a set (a “list”) of TAs to the UE. By ensuring that the centreof this set of TAs is close to the UE's current location, the chance ofa UE rapidly making another tracking area update can be reduced. Ineffect, the TA list (TAL) represents a core network defined area for aUE (referred to as a core network (CN) registration area).

In order to benefit from the lowest energy consumption and to free upvaluable system resources, the UEs return to the RRC idle mode wheneverpossible and perform cell reselections (instead of handovers) as long asthey remain within the same TA. The base station controls the transitionbetween the various operating modes for each UE within its cell(s).Since the setting up and termination of an RRC connection between thebase station and the UE requires exchanging of signalling messages andhence utilises valuable system resources, and also takes some time tocomplete, the transition from connected to idle mode is allowed underspecific circumstances as defined in 3GPP TS 36.331. For example, theserving base station might instruct a UE to enter the RRC idle mode onlyafter it has confirmed that there is no more data to be transmittedto/from the particular UE (e.g. both uplink (UL) and downlink (DL)buffers are empty).

When it registers its current location (e.g. cell) with the corenetwork, each UE also has an associated ‘51’ connection between itsserving base station and the core network. The S1 connection is eitherin a so-called ‘ECM-IDLE’ mode (when the UE is in RRC idle mode) or inan ‘ECM-CONNECTED’ mode (when the UE is in RRC connected mode). The S1connection is used for transferring data (control and user data) betweenthe UE and the core network (and beyond) and it is maintained as long asthe UE remains in the RRC connected mode. On the other hand, when a UEenters the RRC idle mode, its associated S1 connection is alsoterminated (or suspended) until the UE has more data to send or receivein which case a new S1 connection is established to the current servingbase station (or the suspended S1 connection is re-activated).

When the network has data to send to an RRC idle UE, it triggers anappropriate paging procedure in the last known area (tracking/pagingarea) for the UE, which causes the base stations within that area tobroadcast appropriate paging messages in their cells requesting thatparticular UE to enter the RRC connected state. When a previously idlemobile telephone has data to send again (or it has been paged forreceiving downlink data), in order to be allocated communicationresources it initiates a so called RRC connection establishmentprocedure by sending an appropriately formatted RRC connection requestmessage to the base station (following a so-called Random AccessProcedure which ensures that the lower layers, and in particular theMedia Access Control (MAC) layer, are set up for communication with thebase station).

For the latest developments of the 3GPP standards, the so-called NextGeneration (NG) or 5G networks, it is envisaged that UEs may alsooperate in a new RRC state, or new radio state, referred to as an ‘RRCinactive’ state (e.g. in 5G), or a ‘light-connected’ (LC) state/mode(e.g. in LTE/4G). For reasons of simplicity, the term ‘inactive state’will be used to refer to both the 5G RRC inactive state and the LTE/4GLC state/mode.

When a UE is in the inactive state, both the control-plane connection(e.g. over the NG2 reference point in 5G or S1-MME for 4G/LTE) anduser-plane connection (e.g. over the NG3 reference point in 5G or S1-Ufor 4G/LTE) between the RAN (base station) and the core network aremaintained even after the UE has no more data to send or receive (andhence it is normally configured to enter the RRC idle mode). In otherwords, even though in the inactive state the UE is seen as operating inidle mode from the RAN's point of view. It will be appreciated that theinactive state may (or may not) also be transparent to the core network(i.e. seen as being connected from the core network's point of view)even though there is no active RRC connection between the base stationand the inactive state UE. One of the benefits of this new inactivestate is that UEs (IoT devices in particular) that have small andinfrequent data transmissions do not need to perform the entire RRCconnection establishment procedure every time they have data to send (orreceive). Instead, an inactive state capable UE can be configured toresume its existing RRC connection with the current serving base stationwhenever needed and then return to a more power efficient mode ofoperation until it has data to send/receive again.

The UE can resume its RRC connection by sending to its current basestation information (e.g. a resume ID) identifying the connection to beresumed. This beneficially avoids the base station and the UE having togo through authentication and radio bearer establishment. In order tofacilitate such inactive connection and simplified resumption of theconnection between the UE and its serving base station, the concept of aso-called anchor base station is being considered by 3GPP. Effectively,the anchor base station is a base station responsible for storing UEAccess Stratum (AS) context, caching the UE's user data (UE context) andfor providing the user data to other base stations as needed whileterminating the NG core network connections (NG2/NG3). For example, theanchor base station may be the first (or previous) base station that theUE registered with in a particular TA (or other pre-defined area). Thus,when the UE attempts to resume its RRC connection via a different basestation (within the same area), the new base station can contact theanchor base station and retrieve the UE context along with the cacheduser data based on information provided by the UE (e.g. resume ID and/orthe like). Since in the inactive state the NG2/NG3 connections aremaintained, beneficially, the new base station can avoid having tocontact the core network and/or establish new NG2/NG3 connections forthe UE (although the new base station might need to switch the NG2/NG3connections from the anchor/previous base station to the new basestation). This procedure may be referred to as anchor relocation and itinvolves switching an NG2/NG3 terminating points from an Anchor basestation to a new serving base station whilst transferring the UEcontext.

The current agreement in 3GPP is that the base station maintains theNG2/NG3 connections while the UE is in the inactive state and that theRAN (as opposed to the core network) is responsible for initiating anotification procedure for reaching the UE when necessary and forconfiguring the notification related parameters. More specifically, thebase station of the RAN is responsible for notifying the UE when a fullconnection needs to be resumed (e.g. in order to receive downlink datafrom core network) in a paging-like procedure (referred to as RAN-basednotification or RAN-based paging).

In order to facilitate efficient RAN-based notification by a basestation, according to recent developments, a UE in the inactive state(e.g. RRC_INACTIVE) can be configured with a RAN-based notification areathat is a subset of the corresponding core network registration area forthat UE) and which may comprise one or more cells. The RAN-basednotification area is UE-specific and configurable by the base stationvia dedicated signalling. Moreover, direct base station to base stationcommunication via an appropriate interface (e.g. Xn) is availablebetween base stations of the RAN-based notification area. Whilst movingaround (and staying within the boundaries of) this RAN-basednotification area a UE does not need to initiate any procedures toupdate its location with the network (i.e. the does not send any“location update” indication). On leaving the area, however, a UE willupdate its location to the network (e.g. using a location area update ortracking area update procedure). The base station RAN thus remains awarewhenever the UE moves from one RAN-based notification area to another.

SUMMARY OF INVENTION

There are a number of different options for the base station toconfigure the RAN-based notification area. For example, an explicit listof the cell(s) constituting a given RAN-based notification area may benotified to the UE. It will be appreciated that the list may containonly a single entry for implementing a RAN-based notification areacomprising a single cell.

In another example, the RAN-based notification area may be configured(e.g. in the network) as one or more distinct RAN areas each having itsown respective RAN area identifier (RANAID). Each UE may thus beprovided with information identifying one or more RAN Area IDsrepresenting the RAN area(s) (e.g. a list of one or more RAN Area IDs)within which that UE can move without initiating a location area update.To allow the UE to determine whether a particular cell is or is not partof a given RAN area, each cell may broadcast (e.g. in systeminformation) the RAN area ID(s) of the RAN area(s) to which the cellbelongs.

It will be appreciated that one, or both, of these examples may besupported. For example, for high mobility UEs, a base station mayconfigure a list of one or more RAN area IDs representing a RAN-basednotification area within which a UE may remain in an inactive state. Onthe other hand, for low mobility UEs, a base station may configure alist of one or more cells representing a RAN-based notification areawithin which a UE may remain in an inactive state. In this scenario aRAN Area ID broadcast would still be provided by the base station.

An inactive state UE will thus be able to notify the RAN whenre-selecting to a cell not belonging to the configured RAN-basednotification area (e.g. using the resume procedure to notify the RAN ofa RAN-based location area update (RLAU)), in which case the network candecide whether to keep the UE in inactive mode or to ‘suspend’ the UE(e.g. request it to enter RRC idle mode). The RAN-based location areaupdate (RLAU) via the resume procedure may also be triggeredperiodically. In this regard, the connection resume message willgenerally include information that can at least indicate the RAN areaupdate and may include information to enable access control. The UE mayalso perform a CN level location update when crossing a TA boundary whenthe UE is in the inactive state (in addition to RAN updates based on RANareas).

Referring to FIG. 1(a) and FIG. 1(b), planning for tracking areas andRAN areas (where the RAN area example described above is used) isperformed by operators and configured via an operations, administrationand maintenance (OAM) function. Operator planning should guarantee thatany RAN area, of a given RAN-based notification area, is a subset of onetracking area as illustrated in FIG. 1(a). Thus, the scenarioillustrated in FIG. 1(b) in which a RAN area crosses two tracking areasshould be avoided.

Referring to FIG. 2(a) and FIG. 2(b), it is also beneficial to ensurethat the RAN-based notification area is a subset of the core networkregistration area for a given UE as illustrated in FIG. 2(a). However,the inventors have realised that under current proposals it cannot beguaranteed that the RAN-based notification area is a subset of the corenetwork registration area for a given UE and thus, the scenarioillustrated in FIG. 2(b) may arise in which, for a given UE, not all ofthe RAN areas of its RAN-based notification area are within its corenetwork registration area. Specifically, in the example in FIG. 2(b),the RAN-based notification area comprises RAN Areas of RANA3 which ispart of tracking area TA1 and RANA4 and RANA5 which are part of trackingarea TA2. However, TA1 is not part of the TAL for the UE's core networkregistration area that TA2 is part of. Accordingly, the RAN-basednotification area of the UE is not a subset of its core networkregistration areas, which is not ideal. The inventors have understoodthat this issue may occur, for example, because the core networkregistration area (corresponding to the TAL) and the RAN-basednotification area (i.e. list of RAN area(s) when the RAN area exampledescribed above is used) are configured for UEs in separated layers(i.e. the non-access stratum and access stratum layers) and by separatedcommunication entities (i.e. core network node and base station). Thus,whilst the RAN area(s) and tracking area(s) may be planned by operators,the base station may be informed of its cell TAI and RAN area ID, andthe base station may broadcast these identifiers in system information,the base station may, nevertheless, configure a RAN-based notificationarea that overlaps different core network registration areas (i.e. doesnot form a subset of a single core network registration area).

It will be appreciated that although the issue illustrated in FIG. 2(a)and FIG. 2(b), is described in the context of a RAN-based notificationarea formed of plural RAN areas a similar issue may also arise even ifan explicit list of the cell(s) constituting a given RAN-basednotification area is used as opposed to a RAN-based notification areadefined by a list of RAN area identities.

The present invention aims to provide methods and apparatus whichaddress, or at least partially ameliorate, the above issues.

In one example aspect there is provided a method performed by a basestation in a communication system the method comprising: receiving, fromat least one further base station, information identifying at least onerespective tracking area associated with at least one cell of eachfurther base station; receiving, from a core network, informationidentifying a registration area for a user equipment (UE), theregistration area comprising at least a tracking area within which theUE is located; defining, based on the received information identifyingat least one respective tracking area and the received informationidentifying a registration area, a radio access network (RAN) basednotification area for the UE, wherein the RAN based notification arearepresents an area within which the UE can move whilst remaining in aninactive state; and sending, to the UE, information defining the RANbased notification area.

In one example aspect there is provided a method performed by a corenetwork function in a communication system the method comprising:defining, a core network registration area for a user equipment (UE),the registration area comprising at least a tracking area within whichthe UE is located; and providing, to a base station serving the UE,information defining the registration area for the UE.

In one example aspect there is provided a method performed by a userequipment (UE) in a communication system the method comprising:receiving, from at least one base station, information identifying atleast one respective tracking area and the received informationidentifying a registration area, a radio access network (RAN) basednotification area for the UE, wherein the RAN based notification arearepresents an area within which the UE can move whilst remaining in aninactive state; wherein the radio access network (RAN) basednotification area for the UE represents an area defined, based on,information identifying at least one respective tracking area andinformation identifying a registration area, to be a subset of a corenetwork registration area for the UE.

In one example aspect there is provided a base station for acommunication system the base station comprising: a controller and atransceiver, wherein the controller is configured to: control thetransceiver to: receive, from at least one further base station,information identifying at least one respective tracking area associatedwith at least one cell of each further base station; and receive, from acore network, information identifying a registration area for a userequipment (UE), the registration area comprising at least a trackingarea within which the UE is located; define, based on the receivedinformation identifying at least one respective tracking area and thereceived information identifying a registration area, a radio accessnetwork (RAN) based notification area for the UE, wherein the RAN basednotification area represents an area within which the UE can move whilstremaining in an inactive state; and control the transceiver to send, tothe UE, information defining the RAN based notification area.

In one example aspect there is provided a core network function for acommunication system the core network function comprising: a controllerand a transceiver, wherein the controller is configured to: define, acore network registration area for a user equipment (UE), theregistration area comprising at least a tracking area within which theUE is located; and control the transceiver to provide, to a base stationserving the UE, information defining the registration area for the UE.

In one example aspect there is provided a user equipment (UE) for acommunication system the UE comprising: a controller and a transceiver,wherein the controller is configured to control the transceiver toreceive, from at least one base station, information identifying atleast one respective tracking area and the received informationidentifying a registration area, a radio access network (RAN) basednotification area for the UE, wherein the RAN based notification arearepresents an area within which the UE can move whilst remaining in aninactive state; wherein the radio access network (RAN) basednotification area for the UE represents an area defined, based on,information identifying at least one respective tracking area andinformation identifying a registration area, to be a subset of a corenetwork registration area for the UE.

In one example aspect there is provided a base station for acommunication system the base station comprising: means for receiving,from at least one further base station, information identifying at leastone respective tracking area associated with at least one cell of eachfurther base station; means for receiving, from a core network,information identifying a registration area for a user equipment (UE),the registration area comprising at least a tracking area within whichthe UE is located; means for defining, based on the received informationidentifying at least one respective tracking area and the receivedinformation identifying a registration area, a radio access network(RAN) based notification area for the UE, wherein the RAN basednotification area represents an area within which the UE can move whilstremaining in an inactive state; and means for sending, to the UE,information defining the RAN based notification area.

In one example aspect there is provided a core network function for acommunication system the core network function comprising: means fordefining, a core network registration area for a user equipment (UE),the registration area comprising at least a tracking area within whichthe UE is located; and means for providing, to a base station servingthe UE, information defining the registration area for the UE.

In one example aspect there is provided a user equipment (UE) for acommunication system the UE comprising: means for receiving, from atleast one base station, information identifying at least one respectivetracking area and the received information identifying a registrationarea, a radio access network (RAN) based notification area for the UE,wherein the RAN based notification area represents an area within whichthe UE can move whilst remaining in an inactive state; wherein the radioaccess network (RAN) based notification area for the UE represents anarea defined, based on, information identifying at least one respectivetracking area and information identifying a registration area, to be asubset of a core network registration area for the UE.

In one example aspect there is provided a communication systemcomprising: a base station; a core network function; and a UE as set outabove.

In one example aspect there is provided a computer implementableinstructions product comprising computer implementable instructions forcausing a programmable communications device to perform the method asset out above.

Although for efficiency of understanding for those of skill in the art,the example aspects of the invention will be described in detail in thecontext of a 3GPP system (UMTS, LTE, NR), the principles of the exampleaspects of the invention can be applied to other systems in whichcommunication devices or User Equipment (UE) access a core network usinga radio access technology.

Example aspects of the invention extend to corresponding systems,apparatus, and computer program products such as computer readablestorage media having instructions stored thereon which are operable toprogram a programmable processor to carry out a method as described inthe example aspects and possibilities set out above or recited in theclaims and/or to program a suitably adapted computer to provide theapparatus recited in any of the claims.

Each feature disclosed in this specification (which term includes theclaims) and/or shown in the drawings may be incorporated in theinvention independently of (or in combination with) any other disclosedand/or illustrated features. In particular but without limitation thefeatures of any of the claims dependent from a particular independentclaim may be introduced into that independent claim in any combinationor individually.

BRIEF DESCRIPTION OF DRAWINGS

Example embodiments of the invention will now be described, by way ofexample, with reference to the accompanying drawings in which:

FIG. 1(a) and FIG. 1(b), illustrate an issue associated with a radioaccess network (RAN) area that does not properly form a subset of atracking area;

FIG. 2(a) and FIG. 2(b), illustrate an issue associated with theconfigured radio access network (RAN) area list representing aparticular RAN based notification area that does not properly form asubset of the configured tracking area list representing a particularcore network registration area for a given UE;

FIG. 3 illustrates schematically a cellular telecommunication system towhich example embodiments of the invention may be applied;

FIG. 4 illustrates a mechanism for identifying a RAN area that may beimplemented in the system shown in FIG. 3;

FIG. 5 is a block diagram of a UE forming part of the system shown inFIG. 3;

FIG. 6 is a block diagram of a base station forming part of the systemshown in FIG. 3;

FIG. 7 is a block diagram of a core network function forming part of thesystem shown in FIG. 3;

FIG. 8 is a message sequence chart illustrating an exemplary processwhich may be performed by components of the system shown in FIG. 3; and

FIG. 9 is another message sequence chart illustrating an exemplaryprocess which may be performed by components of the system shown in FIG.3.

DESCRIPTION OF EMBODIMENTS <Overview>

FIG. 3 schematically illustrates a telecommunications network 1 in whichitems of user equipment (UEs) 3-1 to 3-4 (such as mobile telephones, andother communication devices, including IoT devices) can communicate witheach other via base stations 5 (in this example gNBs) and a core network7 using an new radio (NR) radio access technology (RAT). As thoseskilled in the art will appreciate, whilst four UEs 3 (denoted ‘UE1’ to‘UE4’) and five base stations 5 a to 5 e are shown in FIG. 3 forillustration purposes, the system, when implemented, will typicallyinclude other base stations and communication devices.

Although not shown in FIG. 3, each base station 5 operates one or moreassociated cells (e.g. base station 5 a operates a ‘Cell A’, basestation 5 b operates a ‘Cell B’, and so on).

UEs 3 can connect to an appropriate cell (depending on their locationand possibly on other factors, e.g. signal conditions, subscriptiondata, capability, and/or the like) by establishing a radio resourcecontrol (RRC) connection with the appropriate base station 5 operatingthat cell. As can be seen, the first UE 3 is located in an area where itcan be served by the cells operated by the base stations 5 a or 5 b.Thus, when operating in RRC idle mode (not sending/receiving data), theUE 3 camps on the cell having the best signal quality, and when in RRCactive mode, the UE 3 communicates data via that cell.

When a UE 3 (e.g. UE3 3-3) first registers with the network (via one ofthe base stations 5), its serving base station 5 (e.g. gNB 5 a) alsoestablishes an associated interface connection for relayingcommunications (user and control data) between the serving base station5 and the core network 7.

The base stations 5 are connected to the core network 7 via NG2/NG3interfaces (or S1-MME/S1-U interface in case of 4G/LTE) and to eachother via an Xn interface (X2 interface in 4G/LTE). The core network 7includes, amongst others, an access and mobility management function(AMF) 7-1 (corresponding generally to a mobility management entity, MMEin 4G/LTE), and a user-plane function 7-2 for providing a connectionbetween the base stations 5 and external networks 15 (such as theInternet) and/or servers hosted outside the core network 7.

The AMF 7-1 is the network node responsible for keeping track of thelocations of the UEs within the communications network 1 especially whena UE 3 is in RRC_IDLE mode. In particular, the AMF 7-1 stores anidentifier of the UEs' last known cell (or tracking area) so that theycan be notified when there is an incoming (voice or data) call for themand that a communication path is set up via the base station 5 currentlyserving the particular UE 3.

In this example, each UE 3 connects to the network periodically (e.g.whenever one of its applications needs to communicate with the network)for sending data to a remote endpoint (e.g. a server or anothercommunication device). Each UE 3 is configured to operate in an RRCinactive state (RRC INACTIVE) in which the network maintains associatedNG2/NG3 connections even though the UE 3 appears to be operating in anidle mode from the RAN's point of view. Therefore, between its periodicre-connections, the UE 3 can enter the inactive mode and thus avoidperforming handovers (i.e. the UE is able to move freely around the RANbased notification area, whilst remaining in the inactive state, withoutthe UE having to update its location with the core network), as long asit remains within a RAN based notification area configured by its anchorbase station 5. It will be appreciated that, whilst use an inactivestate is described in the context of a UE that will have periodical datatransmission this is only exemplary and use of an inactive state inother cases is also relevant.

The base station 5 serving each UE is responsible for configuring anappropriate RAN based notification area for that UE 3. The RAN area maybe configured to comprise one or more cells from the same or differentbase stations 5. In this example, the RAN based notification area forUE3 3-3 is shown as comprising three RAN areas (RANA1, RANA2, andRANA3). RAN area RANA1 includes cells of gNB 5 a and gNB 5 e, RAN areaRANA2 includes the cell or cells of gNB 5 c and RAN area RANA3 includesthe cells of gNB 5 b and gNB 5 d. The core network registration area forUE3 3-3 is configured within the core network (e.g. by the AMF 7-1) andcomprises a tracking area list (TAL) comprising, in this example, twotracking areas TA1 and TA2 where TA1 includes RAN area RANA1 and RANarea RANA2 and TA2 includes RAN area RANA3. It will be appreciated thatthe RAN based notification area and core network registration areaconfigurations shown in and described with reference to FIG. 3 arepurely for illustration and that any suitable configuration is possible.Moreover, it will be appreciated that the configurations are UE specificand may be changed dynamically.

In the example shown in FIG. 3, RAN areas having associated RAN area IDsare configured and, accordingly a RAN based notification area for agiven UE 3 may be configured by providing a list of one or more RAN areaIDs representing that RAN based notification area (althoughconfiguration of a RAN notification area for a given UE using anexplicit lists of cells may also be implemented as an option).

Beneficially, referring to FIG. 4 that illustrates a mechanism foridentifying a RAN area that may be implemented in the system shown inFIG. 3, the RAN areas are uniquely identified by means of a ‘global RANarea ID (RANAID)’ that is a combination of a tracking area identity(TAI) that uniquely identifies the tracking area and a RAN area code(RANAC) which uniquely identifies the RAN area within the tracking arearepresented by the TAI. The TAI is an aggregate identifier comprising apublic land mobile network (PLMN) identity and a tracking area code(TAC) which identifies the tracking area in question within theidentified PLMN. Thus, the same TAC may be used for different PLMNs andthe same RANAC may be used for different tracking areas. As seen in FIG.4, the PLMN identity is also an aggregate identifier comprising a mobilecounty code (MCC) and mobile network code (MNC) for the PLMN. It can beseen, therefore, that any RAN area identified in this way is inherentlywithin (a subset of) a particular tracking and will not overlap morethan one tracking area.

Referring back to FIG. 3, the tracking area to which a base stationbelongs is informed to UEs in its cell(s) by means of broadcast systeminformation (SI—e.g. system information block 1). The broadcast systeminformation comprises a TAC information element and an informationelement comprising a list of one or more PLMNs. The TAC identified bythe TAC information element is common to each PLMN listed. In thisexample, the system information broadcast base station 5 also comprises(in the same or a different system information block) the RAN area codefor the cell.

Thus, the RAN area identity is coded such that the TAI is a part of RANarea identity (e.g. the global RAN area ID={Tracking area ID, RAN areacode within TA}). Since the base station 5 broadcasts the TAC and theRAN area Code within TA, the UE simply needs to interpret that theglobal RAN area ID as being equal to the TAI in combination with the RANarea Code within TA.

Beneficially, neighbouring base stations exchange (e.g. via the Xninterface) cell information with each other, including tracking areainformation (i.e. TAIs for TAs which the cell(s) of the base stationsform part of). Similarly, in this example, RAN area information (e.g.the RAN area ID or RAN area code to be used in conjunction with a TAI)for neighbour cells is provided by the corresponding neighbouring basestations (although it will be appreciated that this information mayalternatively or additionally be provided by an operations,administration and maintenance function (OAM)).

Beneficially, the core network (e.g. a core network control functionsuch as the AMF 7-1) informs the base station of the TA listconfiguration for a given UE (and hence the core network registrationarea configuration for that UE). The information on the TA list may, forexample, be provided during an initial UE context setup procedure and/ormay be provided following a tracking area update.

Thus, when a base station 5 decides to put a UE 3 into an inactive statethe base station can beneficially configure a RAN-based notificationarea for that UE 3 either: using a list of one or more RAN areas whichbelong to tracking area(s) forming part of the tracking list for that UE3; or using an explicit list of one or more cells within trackingarea(s) forming part of the tracking list for that UE 3.

Whilst the example described with reference to FIG. 3 uses the conceptof RAN areas comprising one or more cells, it will be appreciated that asimilar (albeit simpler) procedure may be used beneficially to helpensure that the RAN-based notification area is a subset of the corenetwork registration area for a given UE.

More specifically, if RAN areas are not implemented, the neighbouringbase stations may simply exchange (e.g. via the Xn interface) cellinformation with each other, including tracking area information (i.e.TAIs for TAs which the cell(s) of the base stations form part of)without any RAN area information needing to be provided. As with theprocedure involving RAN areas, the core network (e.g. a core networkcontrol function such as the AMF 7-1) may inform the base station of theTA list configuration for a given UE (and hence the core networkregistration area configuration for that UE). The information on the TAlist may, for example, be provided during an initial UE context setupprocedure and/or may be provided following a tracking area update.

Thus, when a base station 5 decides to put a UE 3 into an inactive statethe base station can beneficially configure a RAN-based notificationarea for that UE 3 using an explicit list of one or more cells withintracking area(s) forming part of the tracking list for that UE 3.

It can be seen, therefore, that by providing the base station withsufficient information to allow the base station to determine whichother cells (and/or RAN areas) form part of a given TA, the aboveprocedures help to ensure that the RAN-based notification area is asubset of the core network registration area for a given UE.

<UE>

FIG. 5 is a block diagram illustrating the main components of the UE 3shown in FIG. 3 (e.g. a mobile telephone or an IoT device). As shown,the UE 3 has a transceiver circuit 31 that is operable to transmitsignals to and to receive signals from a base station 5 via one or moreantenna 33. The UE 3 has a controller 37 to control the operation of theUE 3. The controller 37 is associated with a memory 39 and is coupled tothe transceiver circuit 31. Although not necessarily required for itsoperation, the UE 3 might of course have all the usual functionality ofa conventional mobile telephone (such as a user interface 35) and thismay be provided by any one or any combination of hardware, software andfirmware, as appropriate. Software may be pre-installed in the memory 39and/or may be downloaded via the telecommunications network or from aremovable data storage device (RMD), for example.

The controller 37 is configured to control overall operation of the UE 3by, in this example, program instructions or software instructionsstored within memory 39. As shown, these software instructions include,among other things, an operating system 41, a communications controlmodule 43, a paging module 45, an RRC module 46, and a NAS module 49.

The communications control module 43 is operable to control thecommunication between the UE 3 and its serving base station 5 (and othercommunication devices connected to the serving base station 5, such asother user equipment, core network functions, etc.).

The paging module 45 is responsible for maintaining a RAN basednotification area (e.g. in the form of a list of cells and/or RAN areas)in which the UE 3 can be paged, and to control the transceiver 31 tomonitor for paging/notification messages addressed to the UE 3. Thepaging module 45 is also responsible to notify the other modules (e.g.the RRC module 46 and the NAS module 49, as appropriate) when the UE 3is about to leave (or when it has left) the currently configured RANpaging area (for example, in order to perform an appropriate locationupdate procedure).

The RRC module 46 is operable to generate, send and receive signallingmessages formatted according to the RRC standard. For example, suchmessages are exchanged between the UE 3 and its serving base station 5.The RRC messages may include, for example, messages relating toconfiguring a RAN based notification area for the UE 3.

The NAS module 49 is operable to generate, send and receive signallingmessages formatted according to the NAS standard. For example, suchmessages are exchanged between the UE 3 and the AMF 7-1 (via the servingbase station 5, using the RRC module 46). The NAS messages may include,for example, messages relating to registering and/or updating a trackingarea (or cell) where the UE 3 is currently located.

<Base Station>

FIG. 6 is a block diagram illustrating the main components of a basestation 5 shown in FIG. 3. As shown, the base station 5 has atransceiver circuit 51 for transmitting signals to and for receivingsignals from user equipment (such as the UE 3) via one or more antenna53, a core network interface 55 (e.g. an S1 interface, NG2 and/or NG3interface, and/or the like) for transmitting signals to and forreceiving signals from the core network 7, and a base station interface56 (e.g. an X2 interface, Xn interface, and/or the like) fortransmitting signals to and for receiving signals from neighbouring basestations. The base station 5 has a controller 57 to control theoperation of the base station 5. The controller 57 is associated with amemory 59. Although not necessarily shown in FIG. 3, the base station 5will of course have all the usual functionality of a cellular telephonenetwork base station and this may be provided by any one or anycombination of hardware, software and firmware, as appropriate. Softwaremay be pre-installed in the memory 59 and/or may be downloaded via thecommunications network 1 or from a removable data storage device (RMD),for example. The controller 57 is configured to control the overalloperation of the base station 5 by, in this example, programinstructions or software instructions stored within memory 59. As shown,these software instructions include, among other things, an operatingsystem 61, a communications control module 63, a paging area controlmodule 65, an RRC module 66, a base station signalling module 67, and aCN signalling module 68.

The communications control module 63 is operable to control thecommunication between the base station 5 and the UE 3 (user equipment)and other network entities that are connected to the base station 5. Thecommunications control module 63 also controls the separate flows ofdownlink user traffic (via associated data radio bearers) and controldata to be transmitted to the UEs associated with this base station 5.Control data may include, for example, control data for managingoperation of the UE 3 (e.g. NAS, RRC, paging, system information, and/orthe like).

The paging area control module 65 is responsible for configuring andmaintaining, for each UE for which the base station 5 acts as an anchor,an appropriate RAN based notification area. The paging area controlmodule 65 is also responsible for controlling the transceiver 51 toperform paging/notification of UEs 3, when appropriate (e.g. when thereis downlink data to send to a particular UE 3 that is not in RRCconnected mode). The paging area control module 65 configures the RANbased notification area for a given UE based on information, such astracking area information and/or RAN area information (if appropriate)received from one or more other base stations (e.g. neighbouring basestations) and a tracking area list (TAL) for that UE received from thecore network (e.g. from an AMF 7-1, MME or the like).

The RRC module 66 is operable to generate, send and receive signallingmessages formatted according to the RRC standard. For example, suchmessages are exchanged between the base station 5 and the UE 3 (andother user equipment within the cell of the base station 5). The RRCmessages may include, for example, messages relating to configuring aRAN based notification area for the UE 3.

The base station signalling module 67 is operable to generate, send andreceive signalling messages (X2/Xn messages) formatted according to anappropriate base station to base station application protocol (e.g. anXnAP or an X2AP). The X2/Xn messages may include, for example, messagesrelating to paging a UE 3, data forwarding, transferring/fetching of UEcontext (and other information relating to the UE 3) betweenneighbouring base stations, exchanging information, such as trackingarea information and/or RAN area information (if appropriate) with oneor more other base stations (e.g. neighbouring base stations), etc.

The CN signalling module 68 is operable to generate, send and receivesignalling messages formatted according to an appropriate base stationto core network signalling protocol (e.g. an NG2/NG3 AP or an S1AP). Forexample, such messages are exchanged between the base station 5 and theAMF 7-1/UPF 7-2. The messages may include, for example, messagesrelating to registering the location and/or operating state of userequipment in a cell of the base station 5, obtaining a TA list from thecore network for a particular UE, requesting paging for a particular UE3, and/or associated responses.

<Access and Mobility Management Function>

FIG. 7 is a block diagram illustrating the main components of the accessand mobility management function 7-1 (or similar mobility managemententity) shown in FIG. 3. As shown, the access and mobility managementfunction 7-1 has a transceiver circuit 71 for transmitting signals toand for receiving signals from the base stations 5 (and/or UEs 3connected to the base stations 5) via a base station interface 75 (e.g.an NG2 interface). The access and mobility management function 7-1 has acontroller 77 to control the operation of the access and mobilitymanagement function 7-1. The controller 77 is associated with a memory79. Although not necessarily shown in FIG. 7, the access and mobilitymanagement function 7-1 will of course have all the usual functionalityof a cellular telephone network access and mobility managementfunction/mobility management entity and this may be provided by any oneor any combination of hardware, software and firmware, as appropriate.Software may be pre-installed in the memory 79 and/or may be downloadedvia the communications network 1 or from a removable data storage device(RMD), for example. The controller 77 is configured to control theoverall operation of the access and mobility management function 7-1 by,in this example, program instructions or software instructions storedwithin memory 79. As shown, these software instructions include, amongother things, an operating system 81, a communications control module83, a UE location registration module 85, an NG2 signalling module 88,and a NAS module 89.

The communications control module 83 is operable to control thecommunication between the access and mobility management function 7-1and the base stations 5, the UEs 3, and other network entities that areconnected to the access and mobility management function 7-1.

The UE location registration module 85 is responsible for keeping trackof current location and state (e.g. idle or connected) of user equipmentconnected to the access and mobility management function 7-1.

The NG2 signalling module 88 is operable to generate, send and receivesignalling messages formatted according to an appropriate applicationprotocol for signalling over the NG2 interface between the access andmobility management function 7-1 and the base stations. The NG2 messagesmay include, for example, messages relating to registering the locationand/or operating state of user equipment in a cell of the base station5, providing a TA list to for a particular UE 3 to a base station,requesting paging for a particular UE 3, and/or associated responses.

The NAS module 89 is operable to generate, send and receive signallingmessages formatted according to the NAS standard to and from a UE 3. Forexample, such messages are exchanged between the access and mobilitymanagement function 7-1 and the UE 3 (via a base station 5). The NASmessages may include, for example, messages relating to registeringand/or updating a tracking area (or cell) where the UE 3 is currentlylocated.

In the above description, the UE 3, the base station 5, and the accessand mobility management function 7-1 are described for ease ofunderstanding as having a number of discrete modules (such as thecommunications control modules and the RRC modules). Whilst thesemodules may be provided in this way for certain applications, forexample where an existing system has been modified to implement theinvention, in other applications, for example in systems designed withthe inventive features in mind from the outset, these modules may bebuilt into the overall operating system or code and so these modules maynot be discernible as discrete entities. These modules may also beimplemented in software, hardware, firmware or a mix of these.

A more detailed description will now be given (with reference to FIGS. 5to 10) of various ways in which a RAN based paging area can beconfigured for a UE (which may be operating in LC state/mode).

<Operation—First Example>

FIG. 8 is a message sequence chart illustrating an exemplary processwhich may be performed by components of the system 1 to ensure that aconfigured RAN based notification area for a given UE 3 is a subset of acore network registration area for that UE 3.

As seen at S810, neighbouring base stations 5 exchange cell informationwith each other. The cell information provided by each base station 5comprises respective tracking area information (e.g. the tracking areaidentity) for the cell(s) operated by that base station. The basestations store the information received from other base stations 5 aspart of this exchange. Where RAN areas are used the cell information mayfurther comprise RAN area information (e.g. the RAN area ID or RAN areacode to be used in conjunction with a TAI) for the cell(s) operated bythe corresponding neighbouring base station 5 (although it will beappreciated that this information may alternatively or additionally beprovided by an operations, administration and maintenance function(OAM)).

In this exemplary procedure, information identifying a core networkregistration area for a particular UE is provided using an initial UEcontext procedure as shown between S820 and S828. For example, when a UE3 and a particular base station perform an RRC connection setupprocedure as illustrated at S820, and a base station 5 a sends aninitial UE message (e.g. including an appropriate service request) to acore network function 7 (e.g. the AMF 7-1), the core network functionidentifies the CN registration area for that UE 3 at S823 and respondsto the base station 5 at S824 with an initial context request messageincluding information identifying the CN registration area for the UE 3(e.g. a TA list of one or more TAIs for the TAs of the CN registrationarea). This information is stored at the base station 5 in associationwith information identifying the associated UE. The base station 5 andUE 3 may then engage in an RRC Connection Reconfiguration procedure asappropriate (e.g. as illustrated at S826) and when this is completedsend an appropriately formatted initial context setup response to thebase station at S828 to report to the core network function 7,successful establishment of the security procedures with the UE 3 andthe success/failure of establishment of any radio bearers.

Thus, when a base station 5 decides to put a UE 3 into an inactive stateat S840, the base station 5 can beneficially configure a RAN-basednotification area for that UE 3 at S842 either: using a list of one ormore RAN areas which belong to a tracking area(s) forming part of thetracking list for that UE 3; or using an explicit list of one or morecells within tracking area(s) forming part of the tracking list for thatUE 3.

<Operation—Second Example>

FIG. 9 is a message sequence chart illustrating another exemplaryprocess which may be performed by components of the system 1 to ensurethat a configured RAN based notification area for a given UE 3 is asubset of a core network registration area for that UE 3.

As seen at S910, neighbouring base stations 5 exchange cell informationwith each other in much the same way as illustrated in FIG. 8. Like theprocedure of FIG. 8, the cell information provided by each base station5 comprises respective tracking area information (e.g. the tracking areaidentity) for the cell(s) operated by that base station. The basestations store the information received from other base stations 5 aspart of this exchange. Where RAN areas are used the cell information mayfurther comprise RAN area information (e.g. the RAN area ID or RAN areacode to be used in conjunction with a TAI) for the cell(s) operated bythe corresponding neighbouring base station 5 (although it will beappreciated that this information may alternatively or additionally beprovided by an operations, administration and maintenance function(OAM)).

Unlike the procedure of FIG. 8, however, in this exemplary procedure,information identifying a core network registration area for aparticular UE is provided using an tracking area update (TAU) procedureas shown between S920 and S928. For example, when a UE 3 determines thata tracking area update is required it initiates a tracking area updateby sending, to a core network function 7 (e.g. the AMF 7-1) a trackingarea update request at S920. The core network function 7 obtainsappropriate context information for the UE 3 and determines anappropriate CN registration area for that UE 3 at S922. Assuming thetracking area update request is accepted the core network function 7responds to the UE 3 with an appropriately formatted tracking areaupdate accept message at S924. The UE 3 may, optionally, respond to thetracking area update accept message with an appropriately formattedtracking area update complete message (e.g. if a modified GloballyUnique Temporary ID (GUTI) is included in the tracking area updateaccept message).

Whilst the tracking area updates are sent via the base station they arenon-access stratum (NAS) messages that are transparent to the basestation, accordingly, the base station cannot determine CN registrationarea from the tracking area update accept message. Accordingly,(substantially immediately) following the tracking area updateprocedure, the CN function 7 sends, to the base station 5 CN informationidentifying the CN registration area for the UE 3 (e.g. a TA list of oneor more TAIs for the TAs of the CN registration area). This informationis stored at the base station 5 in association with informationidentifying the associated UE.

Thus, when a base station 5 decides to put a UE 3 into an inactive stateat S940, the base station 5 can beneficially configure a RAN-basednotification area for that UE 3 at S942 either: using a list of one ormore RAN areas which belong to a tracking area forming part of thetracking list for that UE 3; or using an explicit list of one or morecells within tracking area(s) forming part of the tracking list for thatUE 3.

<Modifications and Alternatives>

Detailed example embodiments have been described above. As those skilledin the art will appreciate, a number of modifications and alternativescan be made to the above example embodiments whilst still benefitingfrom the inventions embodied therein. By way of illustration only anumber of these alternatives and modifications will now be described.

It will be appreciated that example embodiments of the invention may beparticularly beneficial for Internet of Things (or machine-type) datatransmissions (e.g. transmission of data acquired during measurementevents and the like). However, it will be appreciated that the exampleembodiments are also beneficially for transmission of any form of datadepending on the application in which the UE is being used. For example,the above example embodiments may be applicable for transmitting datasuch as user data, backup data, synchronisation data, diagnostic data,monitoring data, usage statistics, error data and/or the like.

For simplicity, the base station 5 a is described to be the anchor basestation for each UE (UE1 to UE4) in FIG. 1(a) and FIG. 1(b). However, itwill be appreciated that different UEs may have different associatedanchor base station (even within the same cell) and that the anchor basestation for a given UE may change, when appropriate. In other words,more than one base station may be configured to act as anchor basestation (for one or more UEs), albeit only one anchor base station isused per UE.

In FIG. 3, an Xn interface is provided between two neighbouring basestations and an NG2/NG3 interface is provided between each base stationand the core network. However, it will be appreciated that in othersystems, a different base station to base station interface and/or adifferent base station to core network interface may be provided. Forexample, in 4G/LTE systems the interface between neighbouring basestations is referred to as the ‘X2’ interface and the interface betweena base station and the core network is referred to as the ‘S1’interface. In 4G systems, base stations are referred to as eNBs.

In the above description, when an explicit list of cells (cell IDs) thatbelong to a particular RAN based notification area is used to inform theUE of the RAN based notification area configuration, the list mayinclude the respective tracking area codes/identifiers for each cell. Itwill be appreciated that this allows the UE and the base station touniquely identify cells across multiple tracking areas (in which thesame cell IDs might be reused). However, it will be appreciated that aRAN based notification area may also be defined using a list of cell IDsonly (i.e. without tracking area codes/identifiers).

It will be appreciated that the anchor base station may be configured toconsider neighbour topology and/or (state of) X2/Xn connections with itsneighbour base stations when determining which cells to add/removeto/from the RAN based notification area. For example, neighbour basestation may be configured to exchange information with each other aboutchanges in their configuration (also including changes in their X2/Xnconnection to other base stations) which might require removal of a cellfrom the RAN based notification area (or a RAN area).

It will be appreciated that the RAN base notification area may also bereferred to as a RAN based routing area or a RAN based paging area.

In the above description, the UE, the base station, and the AMF aredescribed for ease of understanding as having a number of discretefunctional components or modules. Whilst these modules may be providedin this way for certain applications, for example where an existingsystem has been modified to implement the invention, in otherapplications, for example in systems designed with the inventivefeatures in mind from the outset, these modules may be built into theoverall operating system or code and so these modules may not bediscernible as discrete entities.

In the above example embodiments, a number of software modules weredescribed. As those skilled in the art will appreciate, the softwaremodules may be provided in compiled or un-compiled form and may besupplied to the base station, to the AMF, to the UE as a signal over acomputer network, or on a recording medium. Further, the functionalityperformed by part or all of this software may be performed using one ormore dedicated hardware circuits. However, the use of software modulesis preferred as it facilitates the updating of the base station, themobility management entity, or the UE in order to update theirfunctionalities.

It will be appreciated that the controllers referred to in thedescription of the UE, the base station, and the AMF (i.e. withreference to FIGS. 5 to 7) may comprise any suitable controller such as,for example an analogue or digital controller. Each controller maycomprise any suitable form of processing circuitry including (but notlimited to), for example: one or more hardware implemented computerprocessors; microprocessors; central processing units (CPUs); arithmeticlogic units (ALUs); input/output (TO) circuits; internal memories/caches(program and/or data); processing registers; communication buses (e.g.control, data and/or address buses); direct memory access (DMA)functions; hardware or software implemented counters, pointers and/ortimers; and/or the like.

Whilst specific hardware apparatus having a specific physical structure(e.g. controllers and transceiver circuitry) have been disclosed forperforming the various procedures described herein, each step of themethods disclosed in the description and/or forming part of the claims,may be implemented by any suitable means for performing that step. Inaccordance with this each example method aspect of the invention has acorresponding example apparatus aspect comprising respective means forperforming each step of that example method aspect.

One example described herein comprises a method performed by a basestation in a communication system the method comprising: receiving, fromat least one further base station, information identifying at least onerespective tracking area associated with at least one cell of eachfurther base station; receiving, from a core network, informationidentifying a registration area for a user equipment (UE), theregistration area comprising at least a tracking area within which theUE is located; defining, based on the received information identifyingat least one respective tracking area and the received informationidentifying a registration area, a radio access network (RAN) basednotification area for the UE, wherein the RAN based notification arearepresents an area within which the UE can move freely, whilst remainingin an inactive state, without the UE needing to update its location withthe core network; and sending, to the UE, information defining the RANbased notification area.

The information identifying a registration area for a UE may bereceived: as part of an initial UE context setup procedure for the UE(e.g. in an initial UE context setup request message; or as part of (orfollowing, e.g. directly after) a tracking area update procedure for theUE. The information identifying at least one respective tracking areaassociated with at least one cell of each further base station may bereceived, for a given further base station, over a direct basestation-to-base station interface or reference point (e.g. and X2 or Xninterface) with that base station.

The method may further comprise receiving information identifying atleast one respective RAN area that at least one cell of each furtherbase station forms part of. The defining of a RAN based notificationarea for the UE may be further based on the received informationidentifying at least one respective RAN area that at least one cell ofeach further base station forms part of. Each RAN area may comprise asubset of at least one cell forming (e.g. representing an area smallerthan) a tracking area associated with that base station. The informationidentifying at least one respective RAN area: may be received, for agiven further base station, over a direct base station-to-base stationinterface or reference point (e.g. and X2 or Xn interface) with thatbase station; and/or may be received from an operations, administrationand maintenance (OAM) function.

The information defining the RAN based notification area: may comprise alist of at least one cell, forming the RAN based notification area, andthat forms at least part of at least one tracking area that forms partof the registration area for the UE; and/or may comprise a list of atleast one RAN area forming the RAN based notification area, wherein eachRAN area in the list respectively comprises at least one cell that formsat least part of a tracking area that forms part of the registrationarea for the UE.

The radio access network (RAN) based notification area may be defined,based on the received information identifying at least one respectivetracking area and the received information identifying a registrationarea, to represent at least one cell that is a subset of cells forming(e.g. to represent an area smaller than) the registration area.

The method may further comprise broadcasting information identifying atleast one respective RAN area that at least one cell of said basestation forms part of. The information identifying at least onerespective RAN area may comprise information identifying at least atracking area code (TAC) identifying a tracking area (e.g. within apublic land mobile network, PLMN) and a RAN area code (RANAC)identifying the RAN area uniquely within the tracking area.

Various other modifications will be apparent to those skilled in the artand will not be described in further detail here.

Some of or all the above-described example embodiments can be describedas in the following Supplementary Notes, but are not limited to thefollowing.

(Supplementary Note 1)

A method performed by a base station in a communication system themethod comprising:

receiving, from at least one further base station, informationidentifying at least one respective tracking area associated with atleast one cell of each further base station;

receiving, from a core network, information identifying a registrationarea for a user equipment (UE), the registration area comprising atleast a tracking area within which the UE is located;

defining, based on the received information identifying at least onerespective tracking area and the received information identifying aregistration area, a radio access network (RAN) based notification areafor the UE, wherein the RAN based notification area represents an areawithin which the UE can move whilst remaining in an inactive state; and

sending, to the UE, information defining the RAN based notificationarea.

(Supplementary Note 2)

A method according to Supplementary Note 1 wherein the informationidentifying a registration area for a UE is received as part of aninitial UE context setup procedure for the UE.

(Supplementary Note 3)

A method according to Supplementary Note 2 wherein the informationidentifying a registration area for a UE is received in an initial UEcontext setup request message.

(Supplementary Note 4)

A method according to Supplementary Note 1 wherein the informationidentifying a registration area for a UE is received as part of (orfollowing, e.g. directly after) a tracking area update procedure for theUE.

(Supplementary Note 5)

A method according to any one of Supplementary Notes 1 to 4 wherein theinformation identifying at least one respective tracking area associatedwith at least one cell of each further base station is received, for agiven further base station, over a direct base station-to-base stationinterface or reference point (e.g. and X2 or Xn interface) with thatbase station.

(Supplementary Note 6)

A method according to any one of Supplementary Notes 1 to 5 furthercomprising receiving information identifying at least one respective RANarea that at least one cell of each further base station forms part of.

(Supplementary Note 7)

A method according to Supplementary Note 6 wherein the defining of a RANbased notification area for the UE is further based on the receivedinformation identifying at least one respective RAN area that at leastone cell of each further base station forms part of.

(Supplementary Note 8)

A method according to Supplementary Note 6 or 7 wherein each RAN areacomprises a subset of at least one cell forming (e.g. representing anarea smaller than) a tracking area associated with that base station.

(Supplementary Note 9)

A method according to Supplementary Note 6, 7 or 8 wherein theinformation identifying at least one respective RAN area is received,for a given further base station, over a direct base station-to-basestation interface or reference point (e.g. and X2 or Xn interface) withthat base station.

(Supplementary Note 10)

A method according to Supplementary Note 6, 7 or 8 wherein theinformation identifying at least one respective RAN area is receivedfrom an operations, administration and maintenance (OAM) function.

(Supplementary Note 11)

A method according to any one of Supplementary Notes 1 to 10, whereinthe information defining the RAN based notification area comprises alist of at least one cell, forming the RAN based notification area, andthat forms at least part of at least one tracking area that forms partof the registration area for the UE.

(Supplementary Note 12)

A method according to any one of Supplementary Notes 1 to 10, whereinthe information defining the RAN based notification area comprises alist of at least one RAN area forming the RAN based notification area,wherein each RAN area in the list respectively comprises at least onecell that forms at least part of a tracking area that forms part of theregistration area for the UE.

(Supplementary Note 13)

A method according to any one of Supplementary Notes 1 to 12 wherein theradio access network (RAN) based notification area is defined, based onthe received information identifying at least one respective trackingarea and the received information identifying a registration area, torepresent at least one cell that is a subset of cells forming (e.g. torepresent an area smaller than) the registration area.

(Supplementary Note 14)

A method according to any one of Supplementary Notes 1 to 13 furthercomprising broadcasting information identifying at least one respectiveRAN area that at least one cell of said base station forms part of.

(Supplementary Note 15)

A method according to Supplementary Note 14 wherein the informationidentifying at least one respective RAN area comprises informationidentifying at least a tracking area code (TAC) identifying a trackingarea (e.g. within a public land mobile network, PLMN) and a RAN areacode (RANAC) identifying the RAN area uniquely within the tracking area.

(Supplementary Note 16)

A method performed by a core network function in a communication systemthe method comprising:

-   -   defining a core network registration area for a user equipment        (UE), the registration area comprising at least a tracking area        within which the UE is located; and providing, to a base station        serving the UE, information defining the registration area for        the UE.

(Supplementary Note 17)

A method performed by a user equipment (UE) in a communication systemthe method comprising:

receiving, from at least one base station, information identifying atleast one respective tracking area and the received informationidentifying a registration area, a radio access network (RAN) basednotification area for the UE, wherein the RAN based notification arearepresents an area within which the UE can move whilst remaining in aninactive state;

wherein the radio access network (RAN) based notification area for theUE represents an area defined, based on, information identifying atleast one respective tracking area and information identifying aregistration area, to be a subset of a core network registration areafor the UE.

(Supplementary Note 18)

A base station for a communication system the base station comprising:

a controller and a transceiver, wherein the controller is configured to:control the transceiver to:

-   -   receive, from at least one further base station, information        identifying at least one respective tracking area associated        with at least one cell of each further base station; and    -   receive, from a core network, information identifying a        registration area for a user equipment (UE), the registration        area comprising at least a tracking area within which the UE is        located;

define, based on the received information identifying at least onerespective tracking area and the received information identifying aregistration area, a radio access network (RAN) based notification areafor the UE, wherein the RAN based notification area represents an areawithin which the UE can move whilst remaining in an inactive state; and

control the transceiver to send, to the UE, information defining the RANbased notification area.

(Supplementary Note 19)

A core network function for a communication system the core networkfunction comprising:

a controller and a transceiver, wherein the controller is configured to:

define a core network registration area for a user equipment (UE), theregistration area comprising at least a tracking area within which theUE is located; and

control the transceiver to provide, to a base station serving the UE,information defining the registration area for the UE.

(Supplementary Note 20)

A user equipment (UE) for a communication system the UE comprising:

a controller and a transceiver, wherein the controller is configured tocontrol the transceiver to receive, from at least one base station,information identifying at least one respective tracking area and thereceived information identifying a registration area, a radio accessnetwork (RAN) based notification area for the UE, wherein the RAN basednotification area represents an area within which the UE can move whilstremaining in an inactive state;

wherein the radio access network (RAN) based notification area for theUE represents an area defined, based on, information identifying atleast one respective tracking area and information identifying aregistration area, to be a subset of a core network registration areafor the UE.

(Supplementary Note 21)

A communication system comprising: a base station according toSupplementary Note 18; a core network function according toSupplementary Note 19, and a UE according to Supplementary Note 20.

(Supplementary Note 22)

A computer implementable instructions product comprising computerimplementable instructions for causing a programmable communicationsdevice to perform the method according to any one of Supplementary Notes1 to 17.

This application is based upon and claims the benefit of priority fromUnited Kingdom Patent Application No. 1712862.0, filed on Aug. 10, 2017,the disclosure of which are incorporated herein in their entirety byreference.

1. A method performed by a radio access network (RAN) node in acommunication system, the method comprising: receiving, from at leastone further RAN node, information identifying at least one respectivetracking area associated with at least one cell of the at least onefurther RAN node; receiving, from an access and mobility managementfunction (AMF), information identifying a registration area configuredfor a user equipment (UE), the information identifying a registrationarea comprising a tracking area list including at least a tracking areaidentifier of a tracking area; configuring, taking account of a UEregistration area, a RAN based notification area for the UE, wherein theRAN based notification area covers at least one cell, wherein the RANbased notification area represents an area within which the UE can movewhilst remaining in an RRC inactive state, wherein the RAN basednotification area is contained within a registration area; receiving,from the UE in the RRC inactive state, a message for indicating a RANbased notification area update; and sending, to the UE after receipt ofthe message for indicating the RAN based notification area update,information for configuring the UE with the RAN based notification area.2. The method as claimed in claim 1, wherein the information identifyinga registration area is received as part of an initial UE context setupprocedure for the UE.
 3. The method as claimed in claim 2, wherein theinformation identifying a registration area for a UE is received in aninitial UE context setup request message.
 4. The method as claimed inclaim 1, further comprising receiving information identifying at leastone respective RAN area.
 5. The method as claimed in claim 4, whereinthe configuring of a RAN based notification area for the UE is furtherbased on the received information identifying at least one respectiveRAN area.
 6. The method as claimed in claim 1, wherein the informationidentifying at least one respective tracking area associated with atleast one cell of the at least one further RAN node is received, fromthe at least one further RAN node, along with a RAN area code.
 7. Themethod as claimed in claim 1, wherein the information identifying atleast one respective RAN area comprises a tracking area identifier (TAI)and, optionally, a radio access network area code (RANAC).
 8. The methodas claimed in claim 1, wherein the information for configuring the UEwith the RAN based notification area comprises an explicit list of atleast one cell that constitute the RAN based notification area.
 9. Themethod as claimed in claim 1, wherein the information for configuringthe UE with the RAN based notification area comprises at least one RANarea identifier of a RAN area.
 10. The method as claimed in claim 1,wherein the RAN based notification area is configured, taking account ofthe UE registration area, to represent at least one cell that is asubset of cells forming the registration area.
 11. A method performed bya user equipment (UE) in a communication system while the UE is in anRRC inactive state, the method comprising: transmitting, to at least oneradio access network (RAN) node, a message for indicating a RAN basednotification area update; and receiving, from the at least one radioaccess network (RAN) node after transmission of the message forindicating the RAN based notification area update, information forconfiguring the UE with a RAN based notification area, wherein the RANbased notification area represents an area configured, taking account ofa registration area for the UE: to cover at least one cell; to becontained within a core network (CN) registration area; and to representan area within which the UE can move whilst remaining in an inactivestate.
 12. A radio access network (RAN) node for a communication systemthe RAN node comprising: a controller and a transceiver, wherein thecontroller is configured to control the transceiver to: receive, from atleast one RAN node, information identifying at least one respectivetracking area associated with at least one cell of the at least onefurther RAN node; receive, from an access and mobility managementfunction (AMF), information identifying a registration area configuredfor a user equipment (UE), the information identifying a registrationarea comprising a tracking area list including at least a tracking areaidentifier of a tracking area; configure, taking account of a UEregistration area, a RAN based notification area for the UE, wherein theRAN based notification area covers at least one cell, wherein the RANbased notification area represents an area within which the UE can movewhilst remaining in an RRC inactive state, wherein the RAN basednotification area is contained within a registration area; receive, fromthe UE (3) in the RRC inactive state, a message for indicating a RANbased notification area update; and control the transceiver to send, tothe UE after receipt of the message for indicating a RAN basednotification area update, information for configuring the UE with theRAN based notification area.
 13. A user equipment (UE) for acommunication system, the UE comprising: a transmitter configured totransmit, to at least one radio access network (RAN) node, a message forindicating a RAN based notification area update; and a receiverconfigured to receive, from at least one radio access network (RAN) nodeafter transmission of the message for indicating the RAN basednotification area update, information for configuring the UE with a RANbased notification area, wherein the RAN based notification arearepresents an area configured, taking account of the registration area:to cover at least one cell; to be contained within a registration areafor the UE; and to represent an area within which the UE can move whilstremaining in an inactive state.